The need to protect electronic circuitry from overvoltages, especially transient overvoltage conditions, is well known. Most electronic components are only built to withstand the application of certain limited voltages across them, and will be damaged or at least seriously malfunction if far higher voltages are applied.
There are many sources of transient overvoltages, such as lightning, electrostatic discharge (ESD), electromagnetic induction (EMI). Failure of circuit components may also allow excess voltages to be applied across other circuit components. Inductive surges are yet another source of overvoltage transients.
Lightning, ESD and inductive surges are all capable of producing very rapid high voltage transients. An inductive surge produced by interrupting a running 115 volt motor can be as high as 1,000 volts or more, for example. Electrostatic discharges, such as those produced by a person walking on a wool rug on a dry winter day, can easily result in a charge of tens of thousands of volts. Although such electrostatic discharges usually involve a relatively minor flow of current, they, like inductive surges, are sufficient to destroy many types of microelectronic circuits. Overvoltage transients caused by lightning can deliver by direct strikes large amounts of currents at tens of thousands to hundreds of thousands of volts. By EMI, lightning can generate high voltage transients in the megahertz frequency range and higher ranges.
Conventional means for dealing with relatively small overvoltages include shunting capacitors, breakdown diodes, varistors and inductive coils. Breakdown diodes such as zener diodes when reverse biased beyond a certain threshold voltage conduct large currents. Like virtually all overvoltage protection devices, such a diode is placed ahead or "upstream" of or in parallel with a circuit element to be protected, and shunts excess voltage applied thereacross to a discharge path such as a neutral line, D.C. common line, chassis or ground. However, such diodes are capable of only handling limited overvoltages without becoming permanently damaged themselves.
Varistors, which are typically made of pressed powders, act somewhat like zener diodes, in that they offer a high impedance at low voltages and a relatively low impedance at high voltages. However, they are distinguished from zener diodes in that their current characteristics are symmetrical rather than asymmetrical, and thus can offer protection against overvoltage in both directions.
Inductive coils or chokes, while unable to protect circuitry from low frequency or static overvoltages, do tend to filter out rapid voltage transients by presenting a large impedance. Since they also present high impedance to high frequency signals, they are inappropriate for protecting high frequency circuitry from high frequency overvoltages. Such chokes are also normally relatively bulky and expensive.
Spark gaps are another form of overvoltage protection associated with higher power devices, and recently miniaturized forms of them have been developed for use on P.C. boards and the like. Spark gaps contain two opposing electrodes separated by a gas, such as air, which has a desired breakdown, or sparking voltage. When an overvoltage is applied across the spark gap, its nonconductive gas becomes ionized, forming a relatively low resistance path between its electrodes. Although spark gaps have beneficial uses, they usually are not very appropriate for use in solid-state circuitry because they are not solid-state devices and because they are usually fairly large, even in miniaturized form. Also the time required for the operation of spark gaps is usually too slow to provide full protection from extremely rapid transients.
Varistors, inductive coils and spark gaps all share the same shortcoming--they cannot be readily incorporated into microelectronic devices due to the required way in which they must be made.
Several types of integrated circuits, CMOS for example, are notoriously sensitive to static electricity, particularly before being inserted into a larger circuit on a P.C. board. Furthermore, the CMOS circuits themselves are typically unable to handle any significant power, so that it is difficult and expensive to arrange on-chip protection by exclusively dedicating certain portions of the chip to such a protection function. Thus, there is a definite need for extremely high speed and/or high power protection that can be readily incorporated directly into all types of microelectronic circuitry, as an integral part thereof, to protect such circuitry at all times.
As a result of the nuclear age a new and very threatening source of overvoltage transients is made possible by the phenomenon known as the nuclear electromagnetic pulse or "EMP". EMP will be produced by the Compton electrons scattered by gamma rays from a nuclear explosion colliding with air molecules of the upper atmosphere. Theoretical studies have indicated that if a nuclear device were exploded at a high altitude above most of the earth's atmosphere a large EMP generated therefrom would have sufficient intensity to induce a large current in conductors hundreds or thousands of miles away to destroy electronic equipment connected to or containg such conductors.
EMP is particularly difficult to protect against for three reasons: (1) the extremely rapid rise time; (2) the expected intensity, and (3) the ubiquitious presence, i.e., all conductors of any appreciable length not enclosed with a suitable Faraday shield will act as an antenna, and thus be subject to severe electrical transients due to the EMP. It has been estimated that EMP will produce an extremely high overvoltage within approximately one nanosecond or less and reach a peak field in only about 10 nanoseconds, before trailing off in about one microsecond. The peak field produced by a one-megaton warhead exploding in the upper atmosphere may be as high as 50,000 volts/meter. Further details about the nature of EMP and the inadequacies of conventional overvoltage protection devices to protect against them is found in "Electromagnetic pulses: potential crippler," IEEE Spectrum, May, 1981, pp. 41-46.
Most conventional solid-state overvoltage protection devices are too slow or limited in their power handling capabilities to provide full protection against the effects of very close lightning strikes or EMP. This is because such lightning strikes and EMP can produce overvoltages two or three orders of magnitude or more above the normal operating voltages of the integrated circuits subjected to such transients, thus leading to enormous current surges capable of destroying virtually almost all types of solid-state semiconductor protection devices. As the energy content of such pulses is increased, the problem becomes more severe, and requires extremely rugged, high ampacity overvoltage protection devices, preferably incorporated at the integrated circuit level, to handle any transients which reach such microelectronic circuits. As the size of microelectronic circuit elements is reduced, the problem also becomes more severe since less energy is required to damage smaller devices. To avoid creating problems, overvoltage protection devices, when inserted into or included as part of the electronic circuit to be protected, must not impose undue insertion losses in the circuit, or decrease switching speeds or band width y adding significant amounts of capacitance.
One class of overvoltage protection devices which has long held great potential for very high speed transient suppression applications are Ovonic threshold switching devices of the type first invented and announced by S. R. Ovshinsky in the 1960's. U.S. Pat. Nos. 3,171,591 (1966) and 3,343,034 to S. R. Ovshinsky (1967) specifically teach that this type of threshold switching device is suitable for use as surge suppressors, such as for transient inductive pulses and the like. Such switches have been known since at least 1968 to have a switching speed of less than 150 picoseconds, see, e.g., S. R. Ovshinsky, "Reversible Electrical Switching Phenomena in Disordered Structures", Physical Review Letters, Vol. 21, No. 20, Nov. 11, 1968, p. 1450(c).
R. Callarotti, et al., "Transmission Line Protection With Thin Film Chalcogenide Glass Devices," Thin Solid Films, Vol. 90, pp. 379-384 (1982), suggest that an Ovonic threshold switch of a thin film of chalcogenide glass is well suited for protecting a transmission line from EMP. A detailed mathematical analysis is presented therein in support of this view.
In U.S. patent application Ser. No. 666,582 filed Oct. 30, 1984 by G. Cheroff et al., which is assigned to the assignee of the present invention, a number of overvoltage protection devices using Ovonic threshold switching materials are proposed. These devices include various electrical connectors with Ovonic threshold switches providing a path for shunting transients to the connector casings, and integrated circuits and printed circuit boards having a thin film of Ovonic threshold material overlying the top wiring layer for providing protection for all conductors forming part of the top wiring layer. These devices are intended for use in protecting against EMP, ESD and other high voltage transients.
Ovonic threshold switching devices may be generally described for the purposes used herein as a switching device which has a bistable characteristic, including a threshold voltage and a minimum holding current. Specifically, the device includes a semiconductor material and at least a pair of electrodes in contact therewith, wherein the semiconductor material has a threshold voltage value and a high electrical resistance to provide a blocking condition for substantially blocking current therethrough, and wherein the high electrical resistance in response to a voltage above the threshold voltage value very rapidly decreases in at least one path between the electrodes to a low electrical resistance which is orders of magnitude lower than the high electrical reistance, which provides a conducting condition or path for conducting current through the semiconductor material. The conducting condition or path is maintained in the device so long as at least a minimum holding current continues to pass through the conducting path within the device. When the current falls below this minimum current value, the device rapidly reverts to its high resistance blocking condition. The voltage drop across the semiconductor material in a threshold switch when in its conducting condition is a fraction of the voltage drop across the material when in its high electrical resistance blocking condition, as measured near the threshold voltage value of the switch.
Many different combinations of atomic elements when combined in the proper proportions and manner have been shown to produce a semiconductor material having the aforementioned threshold switching action. Most commonly, chalcogenide glasses, such as Te.sub.39 As.sub.36 Si.sub.17 Ge.sub.7 P.sub.1, are used. Examples of such materials and threshold switching devices made therewith are found in the following list of U.S. patents, all of which are assigned to the assignee of the present invention, and all of which are hereby incorporated by reference:
______________________________________ 3,271,591 3,571,671 3,343,034 3,571,672 3,571,669 3,588,638 3,571,670 3,611,063 ______________________________________
Threshold switches are generally two terminal devices, and have been shown in a number of configurations, including one having a pair of electrodes arranged in the form of interleaving metallic fingers or combs (see FIG. 7 of U.S. Pat. No. 3,271,591 to S. R. Ovshinsky). Since they exhibit symmetrical current-voltage (I-V) characteristics, have been applied typically in alternating current applications. They are ambipolar devices, that is the current in the conduction path therein consists of both holes and electrons. They can have extremely high current densities. If driven properly, threshold switches can have extremely fast switching speeds, such as into the nanosecond region and below, and make excellent surge suppression devices. Typically, a threshold switch is constructed of a thin film of preferably amorphous semiconductor material, and may be described as a semiconducting glass, although there are a number of other forms of threshold switches such as those described in U.S. Pat. No. 3,715,634 to S. R. Ovshinsky. Two terminal threshold devices, once turned on, cannot be turned off, except by reducing the current through the device below its minimum holding current for the requisite period of time, which is typically well under one microsecond.
The aforementioned patents and patent application, while disclosing a number of useful structures and configurations for Ovonic threshold switching devices in a variety of applications, do not disclose how to optimize the design of such devices for high power, extremely high speed applications. In particular, the foregoing references do not specifically teach any method for avoiding localized concentrations of currents in the threshold switching material which have been known to be of such intensity as to ablate the material or electrodes in contact therewith. The patents also do not teach how to scale up the size of integrated threshold switching devices so that the devices may reliably be used to handle transient currents in excess of several hundred milliamps, such as 5 amps, 10 amps or above.
Accordingly, the objects of the present invention are to provide an overvoltage protection device or apparatus which has at least several of the following attributes: (1) is capable of being scaled up to handle relatively large currents; (2) is of a highly efficient thermal design to allow for dissipation of heat due to the shunting of current produced by extremely large overvoltages; (3) produces minimum insertion losses when in use, and has minimal capacitance; (4) has multiple current paths for shunting current through a threshold switching material, including redundant interconnections to such current paths for increased reliability; (5) is capable of extremely high speed operation; and (6) presents minimum inductance when in use to facilitate such high speed operation.
Another important object of the present invention is to provide and overvoltage device structure which is capable of confining a filamentary current into one or more selected elongated current conduction channels, as a means of obtaining a structure well suited for handling large transient currents and dissipating any heat generated thereby.